Nova API extensions not to be ported to V3

Some of the API extensions which exist in the Nova V2 API are not being ported to the V3 API. The general guideline is that if the extension is just acting as a proxy for another OpenStack service and that same functionality can be requested by the client to that other service, then the extension is not required for V3. An example of this is the os-volumes extension where identical functionality can be requested directly through Cinder.